Re: (Fwd) Your B Series 8088 board reversal

Hi Ruud,

Ed has been helping me get my own 8088 board working. It looks like I might have 
a bad chip or two on my board.
Anyway, I also notice that the xtal at Y1 on my board is 15 MHz, not 12 MHz as 
indicated on your GIF.

Steve



----- Original Message ----
> From: "Ruud@Baltissen.org" <Ruud@Baltissen.org>
> To: cbm-hackers@musoftware.de
> Sent: Sun, December 5, 2010 2:46:04 AM
> Subject: (Fwd) Your B Series 8088 board reversal
> 
> Hallo allemaal,
> 
> 
> I just found this email, very interesting IMHO:
> 
> 
> ------- Forwarded message follows -------
> Send reply to:    <drshock@insectria.net>
> From:    "Edward Shockley" <drshock@insectria.net>
> To:    <ruud@BALTISSEN.ORG>
> Subject:    Your B Series 8088 board reversal
> Date sent:    Sat, 4 Dec 2010 20:55:19 -0500
> 
>     Ruud,
>     Came across your reversal of Bo's 8088 board at 
>     http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/b/8088.txt and 
>the 
>
>     GIF of your schematic.
>     
>     Have been doing a project where I wanted to get the 8087 coprocessor to 
>work on this 
>
>     board for some MS-DOS 1.25 software I felt like porting over. It never 
>worked even back 
>
>     in the day but nobody bothered to figure out why. Anyway, wanted to share a 
>fix for a PCB 
>
>     error that causes the 8087 to hang CP/M-86 or MS-DOS on bootup.
>     
>     In your schematic you show pins 31 and 33 of the 8087 tied together. This 
>is the error that 
>
>     CBM made. The board has a silkscreen for "JP1" in the valley between the 
>8088 and 8087 
>
>     but no PCB jumper was ever installed and only one solder pin hole was 
>drilled for it. This 
>
>     hard link between these two pins must either be cut, or cut and another 
>hole drilled and 
>
>     the jumper installed for the math coprocessor to work. It would be jumpered 
>when not 
>
>     installed, and the jumper removed if installed.
>     
>     The 6509, as you noted, requests the bus from the 8088 across the RQ/GT0 
>line. But only 
>
>     when no 8087 is installed. When an 8087 is added to the circuit the 6509 
>must instead 
>
>     (with the design point the Commodore engineer(s) decided upon) request the 
>bus thru the 
>
>     8087 pin 33 (RQ/GT1) and let the 8087 pin 31 be connected to the 8088 
>RQ/GT0. 
>
>     Connected this way the 8087 will either request the bus for itself, or on 
>behalf of the 6509 
>
>     as necessary. The RQ/GT1 line of the 8088 remains non connected as 
>original.
> 
> If the fix isn't made then when the 8087 is installed the 6509 and 
> the 8087 (during initilization of either CP/M-86 or MS-DOS) will 
> both try to request the bus from the 8088 on the same line and this 
> will lock the bus up and hang the B.
> 
> With this change the 8087 can be installed and function as 
> originally designed without problem - though with it added the 
> board draws over 950mA!
> 
> 
> FYI,
> 
> Edward Shockley
> http://www.insectria.org/b128.html
> 
> ------- End of forwarded message -------
> 
> --
>     ___
>   / __|__
>   / /  |_/    Groetjes, Ruud Baltissen
>   \ \__|_\
>   \___|      http://Ruud.C64.org
> 
> 
> 
> 
> 
> 
> 
>       Message was sent through the cbm-hackers mailing list
> 

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